The grain oriented silicon steel sheet is mainly used as a core of a transformer or other electrical apparatus and is required to have a high magnetic flux density (represented by B.sub.8 value) and a low core loss (represented by W.sub.17/50) as a magnetic property.
In order to improve the magnetic properties of the grain oriented silicon steel sheet, it is required to highly align the &lt;001&gt; axis of secondary recrystallized grain in the steel sheet into the rolling direction on one hand, and it is required to decrease impurities and precipitates remaining in the final product as far as possible on the other hand.
For this end, after a basic production technique of the grain oriented silicon steel sheet through two-stage cold rolling has been proposed by N. P. Goss, many improvements for such a production technique have been repeated to improve the magnetic flux density and core loss value of the grain oriented silicon steel sheet every year.
Among them, there are typically a method described in JP-B-51-13469 using Sb and MnSe or MnS as an inhibitor and a method described in JP-B-33-4710, JP-B-40-15644, JP-B-46-23820 and the like using AlN and MnS as an inhibitor. According to these methods, there was obtained a product having a high magnetic flux density that B.sub.8 exceeds 1.88T.
In order to obtain a product having a higher magnetic flux density, JP-B-57-14737 discloses the composite addition of Mo to a starting material or JP-B-62-42968 discloses the application of quenching treatment after the intermediate annealing just before final cold rolling after the composite addition of Mo to the starting material, whereby there are obtained a high magnetic flux density where B.sub.8 is not less than 1.90T and a low core loss that core loss W.sub.17/50 is not more than 1.05 W/kg (product thickness: 0.30 mm). However, there is left room to be further improved as to sufficient reduction of core loss.
Particularly, it is considerably demanded to reduce power loss as far as possible because of the energy crisis, and it is desired to more improve the loss even in the application as an iron core material accompanied therewith. For this end, many products thinning the product thickness to not more than 0.23 mm (9 mil) are used for decreasing eddy current loss as much as possible.
The aforementioned techniques are mainly metallurgical methods. Besides these methods, there is developed a method of reducing core loss (technique of finely dividing magnetic domain), in which the surface of the steel sheet after the final annealing is subjected to laser irradiation or plasma irradiation to artificially decrease the 180.degree. magnetic domain width (B. Fukuda, K. Sato, T. Sugiyama, A. Honda and Y. Ito: Proc. of ASM Con. of Hard and Soft Magnetic Materials, 8710-008, (USA), (1987)). The core loss of the grain oriented silicon steel sheet is largely reduced by the development of such a technique.
However, this technique has a drawback that it is not durable to annealing at a higher temperature, so that there is a problem that the application is restricted to only a laminated core type transformer not requiring strain relief annealing.
In this connection, a method wherein linear grooves are introduced in a surface of a steel sheet after the final annealing of the grain oriented silicon steel sheet to finely divide magnetic domain through anti-magnetic field effect of such grooves is industrialized as a finely magnetic domain dividing technique durable to strain relief annealing (H. Kobayashi, E. Sasaki, M. Iwasaki and N. Takahashi: Proc. SMM-8., (1987), P.402).
Besides this technique, a method wherein the magnetic domain is divided by subjecting a final cold rolled sheet of the grain oriented silicon steel sheet to a local electrolytic etching to form grooves (JP-B-8-6140) is also developed and industrialized.
Apart from the aforementioned production methods of the silicon steel sheet, amorphous alloys are noticed as a material for the usual power transformer, high-frequency transformer or the like as disclosed in JP-B-55-19976, JP-A-56-127749 and JP-A-2-3213.
However, a very excellent core loss property is obtained in such amorphous materials as compared with the conventional grain oriented silicon steel sheet, but they have demerits in practical use because thermal stability is lacking, space factor is poor, cutting is not easy, and they are too thin and brittle, to bringing about a large cost up in the assembled step of the transformer, and hence it is not yet attained to use a greater amount of such materials at the present time.
In addition, JP-B-52-24499 proposes a method wherein a forsterite base film formed after the final annealing of the silicon steel sheet is removed and the surface of the steel sheet is polished and then the surface of the steel sheet is subjected to a metal plating.
In this method, however, a low core loss is obtained at a low temperature, but when it is subjected to a high temperature treatment, the metal diffuses into the silicon steel sheet and there is a drawback that the core loss property is rather degraded.
In order to solve the above problem, the inventors have disclosed in JP-B-63-54767 and the like that an ultra-low core loss is obtained by forming one or more tension films selected from the group consisting of nitrides and carbides of Si, Mn, Cr, Ni, Mo, W, V, Ti, Nb, Ta, Hf, Al, Cu, Zr and B on the grain oriented silicon steel sheet smoothened by polishing through CVD or a dry plating (PVD) such as ion plating, ion implantation or the like.
Although a very excellent core loss property as a material for a power transformer, high-frequency transformer or the like is obtained by such a production method, it can not be said to sufficiently respond to the recent demand for the attainment of low core loss.
Therefore, the inventors have made fundamental reexaminations from all viewpoints for more reducing the core loss as compared with the conventional one.
That is, in order to obtain product having a ultra-low core loss by forming one or more tension films selected from various nitrides and carbides on the smoothened surface of the grain oriented silicon steel sheet at a stabilized step, the inventors became aware that it is required to conduct fundamental reexamination from raw material components of the grain oriented silicon steel sheet to the final treating step, and have made various studies from a pursuit on texture of a silicon steel sheet to smoothness of steel sheet surface or a final CVD or PVD treating step.
As a result, we have found the following:
(1) A thin ceramic film covered on the silicon steel sheet (use TiN film as a typical example) lessens the degree of improving the core loss even when it is formed at a thickness of not less than 1.5 .mu.m. That is, TiN film having a thickness of not less than 1.5 .mu.m can expect a slight improvement to the core loss and rather brings about the degradation of space factor and magnetic flux density. PA1 (2) In this case, TiN is more important to play a role of adhesion to the silicon steel sheet in addition to the application of tension inherent to the ceramic. That is, when a lateral section of TiN is observed by means of a transmission electron microscope (see Yukio Inokuti: Bulletin of The Japan Institute of Metals, 60(1996), P.781.about.786), a lateral stripe of 10 nm is observed, which corresponds to a 5 atom layer of Fe--Fe atom in [011] direction of the silicon steel sheet. PA1 (3) When a two-layer texture of a TiN covered zone and chemical polished zone is simultaneously measured by X-ray (see Y Inokuti: ISIJ International, 36(1996), P.347.about.352), the {200} peak form of Fe in the polished zone is an circle. However, the {200} peak form of Fe in the TiN covered zone is an ellipsoid and is at a state of strongly applying tension in the [100].sub.si-steel direction of the silicon steel sheet. PA1 (4) Tension of TiN film is 8.about.10 MPa (see Yukio Inokuti, Kazuhiro Suzuki, Yasuhiro Kobayashi: Bulletin of The Japan Institute of Metals, 60(1996), P.674.about.678), from which an improvement of magnetic flux density of about 0.014.about.0.016T can be expected. (This corresponds to the improvement of degree of Goss orientation alignment of about 1.degree.). PA1 (5) When the final cold rolled sheet of the silicon steel sheet is subjected to local electrolytic etching to form grooves and the surface of the steel sheet after the secondary recrystallization treatment is smoothened by polishing a TiN ceramic film is coated thereon, the core loss is effectively reduced by fine division of magnetic domain through the anti-magnetic field that resulted from the formed grooves and further by the application of tension through the ceramic film. PA1 (6) The effect of reducing the core loss by tension when a concave groove is formed on the surface of the steel sheet prior to the ceramic coating is larger than that of the silicon steel sheet smoothened by the usual polishing (see JP-B-3-32889). PA1 (7) When the ceramic film is coated on the silicon steel sheet having the concave grooves therein, the effect of reducing the core loss is more effective than the case of smoothening by polishing and coating the ceramic film. PA1 (8) When the grooves are formed by subjecting the final cold rolled sheet of the silicon steel sheet to local electrolytic etching, even if a TiN ceramic film is formed at a surface state that the surface of the steel sheet after the secondary recrystallization treatment is not smoothened by polishing, a considerable effect of reducing the core loss is developed. That is, when the ceramic film having a small thermal expansion coefficient is coated even at a state of being not smoothened by polishing, e.g. with small irregularities on the surface through pickling treatment or the like, it is possible to apply a strong tension to the surface of the silicon steel sheet, whereby the core loss can advantageously be reduced. PA1 1 An etching resist ink consisting essentially of an alkyd resin is applied onto the surface of the final cold rolled sheet by gravure offset printing so as to leave linear non-coated portions of width: 200 .mu.m at an interval: 4 mm in a direction substantially perpendicular to a rolling direction, and baked at 200.degree. C. for 3 minutes. In this case, a resist thickness is 2 .mu.m. The steel sheet coated with the etching resist is subjected to an electrolytic etching to form linear grooves of width: 200 .mu.m and depth: 20 .mu.m and then immersed in an organic solvent to remove the resist. In this case, the electrolytic etching is carried out in NaCl electrolyte under conditions of current density: 10 A/dm.sup.2 and treating time: 20 seconds. PA1 2 For the comparison, there is provided the final cold rolled sheet not subjected to the treatment of the item 1. PA1 (A) After an extremely thin Si film of about 0.02 .mu.m in thickness is formed on the surface of the silicon steel sheet by magnetron sputtering process (one of PVD processes), it is treated in a mixed gas of N.sub.2 (50%)+H.sub.2 (50%) at 1000.degree. C. for 10 minutes. Thereafter, a tension insulating film (thickness of about 2 .mu.m) consisting essentially of colloidal silica and phosphate is formed on the surface of the steel sheet and baked at 800.degree. C. PA1 (B) The surface of the silicon steel sheet is treated in a mixed gas of SiCl.sub.4 +N.sub.2 +H.sub.2 at 950.degree. C. for 10 minutes (CVD process). Thereafter, a tension insulating film (thickness of about 2 .mu.m) consisting essentially of colloidal silica and phosphate is formed on the surface of the steel sheet and baked at 800.degree. C. PA1 (C) The silicon steel sheet is immersed in an aqueous solution of SiCl.sub.4 (0.5 mol/1) at 80.degree. C. for 10 seconds and treated in a mixed gas of N.sub.2 (50%)+H.sub.2 (50%) at 900.degree. C. for 10 minutes. Thereafter, a tension insulating film (thickness of about 2 .mu.m) consisting essentially of colloidal silica and phosphate is formed on the surface of the steel sheet and baked at 800.degree. C. PA1 1 An etching resist ink consisting essentially of an alkyd resin is applied onto the surface of the final cold rolled sheet by gravure offset printing so as to leave linear non-coated portions of width: 200 .mu.m at an interval: 4 mm in a direction substantially perpendicular to a rolling direction, and baked at 200.degree. C. for 3 minutes. In this case, a resist thickness is 2 .mu.m. The steel sheet coated with the etching resist is subjected to an electrolytic etching to form linear grooves of width: 200 .mu.m and depth: 20 .mu.m and then immersed in an organic solvent to remove the resist. In this case, the electrolytic etching is carried out in NaCl electrolyte under conditions of current density: 10 A/dm.sup.2 and treating time: 20 seconds. PA1 2 For the comparison, there is provided the final cold rolled sheet not subjected to the treatment of the item 1. PA1 (a) The oxide film on the surface of the silicon steel sheet treated under the condition 1 is treated with a mixed pickling solution of HCl(10%) and H.sub.3 PO.sub.4 (8%), immersed in an aqueous solution of SiCl.sub.4 (0.02 mol/l) at 85.degree. C. for 30 seconds and then a tension insulating film (thickness of about 1.5 .mu.m) consisting essentially of magnesium phosphate and colloidal silica is formed (800.degree. C.) on the surface of the steel sheet. PA1 (b) After the oxide film on the surface of the silicon steel sheet treated under the condition 1 is treated with HCl(10%), it is chemically polished with 3% hydrofluoric acid and hydrogen peroxide, immersed in an aqueous solution of SiCl.sub.4 (0.02 mol/l) at 85.degree. C. for 30 seconds and then a tension insulating film (thickness of about 1.5 .mu.m) consisting essentially of magnesium phosphate and colloidal silica is formed (800.degree. C.) on the surface of the steel sheet. PA1 (c) On the surface of the silicon steel sheet provided with forsterite film treated under the condition 2 is formed (800.degree. C.) a tension insulating film (thickness of about 1.5 .mu.m) consisting essentially of magnesium phosphate and colloidal silica. PA1 1 An etching resist ink consisting essentially of an alkyd resin is applied onto the surface of the final cold rolled sheet by gravure offset printing so as to leave linear non-coated portions of width: 200 .mu.m at an interval: 4 mm in a direction substantially perpendicular to a rolling direction, and baked at 200.degree. C. for 3 minutes. In this case, a resist thickness is 2 .mu.m. The steel sheet coated with the etching resist is subjected to an electrolytic etching to form linear grooves of width: 200 .mu.m and depth: 20 .mu.m and then immersed in an organic solvent to remove the resist. In this case, the electrolytic etching is carried out in NaCl electrolyte under conditions of current density: 10 A/dm.sup.2 and treating time: 20 seconds. PA1 2 For the comparison, there is provided the final cold rolled sheet not subjected to the treatment of the item 1. PA1 (A) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water and further adding 25 cc of SiCl.sub.4 solution to the diluted solution for 20 seconds, washed with water and dried. PA1 (B) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water and further adding 25 cc of SiCl.sub.4 solution and 25 g of FeCl.sub.3 together to the diluted solution for 20 seconds, washed with water and dried. PA1 (C) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water and further adding 25 cc of SiCl.sub.4 solution and 25 g of AlPO.sub.4.3/2H.sub.2 O together to the diluted solution for 20 seconds, washed with water and dried. PA1 (D) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water and further adding 20 g of FeCl.sub.3, 20 g of Al(NO.sub.3) and 10 g of H.sub.3 BO.sub.3 together to the diluted solution for 20 seconds, washed with water and dried. PA1 (E) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water for 20 seconds, washed with water and dried. PA1 (F) The silicon steel sheet is immersed in a treating solution of 80.degree. C. obtained by diluting 250 cc of a coating solution for tension insulating film consisting essentially of phosphate and colloidal silica with 1500 cc of distilled water and further adding 25 cc of SiCl.sub.4 solution to the diluted solution for 20 seconds, washed with water and dried. PA1 (G) After the final annealing, the oxide on the surface of the silicon steel sheet is removed by pickling.
Although the above is novel knowledge regarding the ceramic coating, we have further obtained the following knowledge relating to surface state of ceramic film and steel sheet.
That is, when the groove is formed, a difference between tension through the coating on the groove forming portion and tension through the coating on the portion not forming the groove or a different tension is applied to the surface of the silicon steel sheet to increase the degree of reduction of the core loss by such a tension.
That is, the linear grooves are formed to finely divide the magnetic domain through the anti-magnetic effect of these grooves and then the ceramic tension film is formed to further finely divide 180.degree. main magnetic domain, whereby ultra-low core loss is more effectively obtained.
We have made many experiments and examinations based on the above knowledge in order to achieve a given object and found out that it is very effective to reduce the core loss when plural kinds of ceramic tension films are formed on the surface of the silicon steel sheet in either case of the surface-smoothened silicon steel sheet and the linear groove-formed silicon steel sheet, and the thermal expansion coefficients of these ceramic tension films are decreased toward the outside, and grain oriented silicon steel sheets having a very low core loss are newly developed (specification of Japanese Patent Application No. 9-328042).
The thus obtained grain oriented silicon steel sheets are provided with a very thin ceramic tension film having an excellent adhesion property and are possible to attain an ultra-low core loss and have an insulating property and are excellent in the space factor, so that they are certainly said to be ideal silicon steel sheets.
However, treatment in a high plasma atmosphere under vacuum is indispensable for forming such a dense ceramic film. In this case, the ceramic film can not be formed at a high speed and productivity is low, so that there is a problem that the cost is up.
Besides this, Japanese Patent No. 2662482 and No. 2664326 recently proposed low core loss grain oriented silicon steel sheets having improved adhesion to film and core loss by forming a composite film of oxidized Al-oxidized B on the smoothened surface of the steel sheet.
However, the core loss value W.sub.17/50 of the silicon steel sheet formed by these methods is only about 0.77.about.0.83 W/kg in a product having a thickness of 0.2 mm, so that it should be said that there is left room to be improved because the core loss value is merely related to the extent the product thickness is thinned.